The Facebook preview uses the HTML page header "description" meta tag to generate the text to display when you post an URL. For the geocaching.com.au URLs that is "Geocaching is a treasure hunt using a GPS enabled device to find a geocache near your home, city, the bush, anywhere in the world. Beware it can be addictive!".

This is a pretty generic text and doesn't say anything about the page itself. When I share a cache page, it doesn't give any information about the cache, it could for example give information about the cache name, code and location. For the xmas in July page and its friends, it can give some information about that game.

Changing this will draw the attention of the readers more than the generic text about geocaching.